1. Field of the Invention
This invention relates generally to laser systems, and more particularly, to a laser device which relies upon energy-sharing interactions between atomic quartets, whereby four-fold continuous-wave laser action is produced.
2. Description of the Prior Art
There is a need for short-wavelength operation of optically-excited, solid state and other lasers. Short wavelength lasers, particularly solid state lasers which are compact and efficient, have considerable promise for applications to high density optical disk storage as well as various display and communications markets. Although some forms of earlier upconversion lasers address the need in the art, to an extent, these known arrangements fall short of ideal performance, as there is a need for a laser system which achieves much high upconversion factors. It is believed that a significant improvement over preexisting upconversion laser systems would require a new, and consequently important, mechanism for achieving very short wavelength solid state laser sources.
Current compact disks use infrared diode lasers for reading and writing information on storage media. Much shorter wavelengths can be produced using upconversion processes which are the basis of the current invention. Previous upconversion lasers and patents on the topic refer only to pair or trio processes for the achievement of the population inversion which is essential to obtain laser action. Prior publications exist regarding the achievement of laser action based on multiphoton absorption, avalanche pumping and cooperation upconversion. However, no previous upconversion laser of any description has achieved continuous-wave laser action from states lying about the ground state by more than three times the incident photon energy.
There is, therefore, a need for a continuous-wave laser system wherein laser action is achieved from energy states lying at least four times the incident energy per photon above the ground state. A laser system which would achieve this result is expected to make practical a variety of new, short wavelength solid state lasers (ultraviolet wavelengths) which have previously not operated at all or have not operated efficiently because of the conventional need for pump sources with more energy per photon than the emitting state of the laser transition. Pair-pumped solid state lasers can operate at wavelengths as short as one half that of the pump light.
As previously noted, short wavelength solid state lasers, if sufficiently efficient, should be important for improved compact disk storage technology. This is because shorter wavelengths make smaller focal spot sizes possible. Hence, increased storage density becomes possible on a disk of given area. (Of the various categories of convention lasers which exist, solid state lasers lend themselves best to miniaturization and low total power consumption. these factors are additional, key aspects of their significance for commercialization.)
Existing compact disk technology relies on GaAs-GaAlAs diode lasers operating at a wavelength of 800 nm in the near infrared region. A pair laser pumped by light of this wavelength could operate at wavelengths as short as 400 nm, resulting in an improvement in storage capacity of a factor of four. This nonlinear increase in storage capacity result from the fact that spot radius is proportional to wavelength but storage density depends on the spot area, proportional to the square of the radius. Hence, significant improvements in storage can result from small decreases in operating wavelength.
There are currently no blue or ultraviolet solid state lasers which operate continuously and have been developed commercially for compact disk applications or other technologies, such as space communications. The present invention will provide to the art a new mode of laser excitation that will permit the development of new short wavelength solid state lasers for which there has previously been no suitable excitation mechanism. For emission wavelengths short enough to approach the fundamental absorption edge of solids, it is not possible to deliver conventional excitation at still shorter wavelengths without severe heating and damage of the solid, and very low efficiencies. Pump light cannot even penetrate to the interior of the lasing medium. By contrast, upconversion-pumping permits excitation at longer wavelengths than either the emission wavelength of the energy of the emitting state with respect to the ground state. Hence, penetration of the pump light is not a problem, and deleterious color center formation can be avoided.
There is no prior art known to Applicants describing continuous-wave solid state lasers in which pumping is provided by four-fold or higher upconversion. Prior art does exist for two-fold and three-fold upconversion lasers in several Er-doped crystals. However, there is no suggestion in the known art that inversion can be sustained continuously by such unlikely processes as those requiring the absorption of four or more photons, or the energy-pooling of four or more initially excited ions, or some combination of these mechanisms.
It is, therefore, an object of this invention to provide a laser system wherein short-wavelength emission is achieved in response to longer-wavelength optical excitation of solid state laser media.
It is another object of this invention to provide a short-wavelength laser system which does not require pump sources with more energy per photon than the emitting state of the laser transition.
It is also an object of this invention to provide a laser arrangement which achieves a significantly reduced focal spot size.
It is a further object of this invention to provide a laser arrangement which improves the storage density of a compact disk by a factor of four.
It is additionally an object of this invention to provide a laser system which can operate at wavelengths on the order of 700 nm.
It is yet a further object of this invention to provide a laser arrangement which is adaptable for miniaturization, and which is efficient in its energy utilization.
It is also another object of this invention to provide a short-wavelength solid state laser which can readily be miniaturized.
It is yet an additional object of this invention to provide a blue or ultraviolet continuous-wave solid state laser.